The invention concerns from a hydraulic control circuit, by means of which a primary first hydraulic consumer and a secondary second hydraulic consumer can be supplied with pressure medium in general 1.
Such a hydraulic control circuit is known from DE 43 28 283 A1. In this, the pressure medium flows to the two hydraulic consumers in each case via a metering diaphragm, the first metering diaphragm assigned to the primary first hydraulic consumer being preceded by a pressure compensator, and the second metering diaphragm assigned to the secondary second hydraulic consumer being followed by a pressure compensator. With the aid of the pressure compensators, if a sufficient quantity of pressure medium is delivered, constant pressure differences are maintained via the metering diaphragms, irrespective of the load pressures of the hydraulic consumers, so that the pressure medium quantity flowing to a hydraulic consumer depends only on the opening cross section of the respective metering diaphragm. An adjustable hydraulic pump usually serves as a pressure medium source and is capable of being controlled as a function of the highest load pressure in such a way that the pressure in an inflow line is above the highest load pressure by the amount of a specific pressure difference. The pressure compensator following the second metering diaphragm is acted upon in the opening direction by the pressure downstream of the second metering diaphragm and in the closing direction by a control pressure which prevails in a rear control space and which usually corresponds to the highest load pressure of all the hydraulic consumers supplied by the same hydraulic pump. If a plurality of hydraulic consumers, to which pressure medium flows in each case via a metering diaphragm and a pressure compensator which follows the latter and which is acted upon at the rear by the highest load pressure, are actuated simultaneously, the pressure medium quantities flowing to them are reduced in equal ratio if the pressure medium quantity delivered by the hydraulic pump is lower than the pressure medium part quantities required. A control with load-independent throughflow distribution (LUDV control) is referred to in this case. Hydraulic consumers controlled in this way are called, in brief, LUDV consumers. Since, in an LUDV control, the highest load pressure is also sensed and an inflow pressure lying above the highest load pressure by the amount of a specific .DELTA.p is generated by the pressure medium source, an LUDV control is a special instance of a load-sensing control (LS control).
There is no load-independent throughflow distribution in the case of a plurality of hydraulic consumers, to which pressure medium flows in each case via a metering diaphragm with a preceding pressure compensator which is acted upon in the closing direction only by the pressure upstream of the metering diaphragm and in the opening direction only by the load pressure of the respective hydraulic consumer and by a compression spring. Only an LS control and an LS consumer are available. DE 43 28 283 A1, then, discloses priority switching between an LS consumer and one or more LUDV consumers, in which the LS consumer is supplied as primary consumer with pressure medium. For this purpose, a priority valve is provided, which has a first connection, connected to a line section upstream of the first metering diaphragm, and a second connection, connected to the load signaling line, and the valve member of which is capable of being acted upon, in the direction of the opening of the connection between the first connection and the second connection, by the load pressure of the primary hydraulic consumer, that is to say of the LS consumer, and by an additional force. The priority valve in the control according to DE 43 28 283 A1 is acted upon, in the direction of the closing of the connection between the first connection and the second connection, by the pressure in the second connection. Although this ensures that the LS consumer is supplied as primary consumer with pressure medium, the pressure in the inflow line is unnecessarily high in specific situations, so that power losses occur. Such a situation arises, for example, when the load pressure of the primary hydraulic consumer is higher than the load pressure of the secondary hydraulic consumer. A pressure lying above the load pressure of the primary hydraulic consumer by the amount of a pressure difference equivalent to the additional force acting on the valve member of the priority valve is then built up in the load signaling line. The regulation of the hydraulic pump, in turn, gives rise, in the inflow line, to a pressure lying above the pressure in the load signaling line by the amount of a specific .DELTA.p, so that the pressure in the inflow line lies above the load pressure of the primary hydraulic consumer by an amount more than the regulating .DELTA.p at the regulating member of the hydraulic pump.
While a priority control between an LS consumer and an LUDV consumer is disclosable by DE 43 28 283 A1, DE 35 07 122 C2 shows a priority control between two LS consumers. A pressure medium quantity thus flows to these two hydraulic consumers in each case via a metering diaphragm and a pressure compensator which precedes this metering diaphragm and which is acted upon in the closing direction by the pressure upstream of the metering diaphragm. The pressure compensator which is assigned to the primary hydraulic consumer is acted upon in the opening direction by the load pressure of this hydraulic consumer and by a compression spring. The pressure compensator for the secondary hydraulic consumer is acted upon in the closing direction likewise by a compression spring and, moreover, by a pressure picked off between a fixed throttle and a proportional diaphragm which serves as a priority valve and which is connected between the fixed throttle and a tank line and is controlled by the pressure difference at the metering diaphragm of the primary hydraulic consumer. In the event of undersaturation, that is to say when an insufficient quantity of pressure medium is conveyed, the pressure difference at the metering diaphragm of the hydraulic consumer to be supplied as primary consumer with pressure medium decreases, so that the proportional diaphragm opens somewhat, the pressure between the latter and the fixed throttle falls somewhat and the pressure compensator of the hydraulic consumer to be supplied as secondary consumer with pressure medium closes until sufficient pressure medium is available again for the primary hydraulic consumer.